CODEWITHSUNDEEP
jsongoreflection
Ewan Valentine
Ewan Valentine

Reputation: 3941

Is it possible to dynamically set the output of json.Unmarshal?

I have the following Go code:

var typeRegistry = make(map[string]reflect.Type)

func init() {
    typeRegistry["User"] = reflect.TypeOf(User{})
}

func makeInstance(name string) interface{} {
    v := reflect.New(typeRegistry[name]).Elem()
    return v.Interface()
}

func Invoke(any interface{}, name string, body []byte, signature Signature) {
    args := signature.Args
    data := makeInstance(signature.Args[0])
    json.Unmarshal(body, &data)
    inputs := make([]reflect.Value, len(args))
    for i, _ := range signature.Args {
        log.Println(reflect.TypeOf(data))
        log.Println(reflect.ValueOf(data))
        inputs[i] = reflect.ValueOf(data)
    }
    reflect.ValueOf(any).MethodByName(name).Call(inputs)
}

I'm attempting to pass in some JSON, and a string denoting what type the JSON should be mapped to. I'm attempting to use reflection to mush the two together again and pass it into a method by the methods name.

I kind of got it working, however, when I use a pointer un json.Unmarshal it seems to lose the reference to its assigned type again, and defaults back to map[string]interface{} which is a mis-match for the method I'm calling. In this case it's expecting type main.User. If I remove the pointer from json.Unmarshal(body, data) the types match correctly, but obviously the data is no longer being set for data.

I'm aware I'm bastardising Go's type-system, and probably using the language in ways that isn't suggested, but I'm trying to do something more academic than useful, I guess.

Upvotes: 2

Views: 94

Answers (1)

icza
icza

Reputation: 417702

You have to pass a pointer to json.Unmarshal(), else it can't change the (pointed) data, only a copy of it. This is what happens when you pass data: since it can't change the value (non-pointer value of type User wrapped in interface{}), therefore it creates a new value it can unmarshal into. And it will choose whatever it sees fit bests (but it won't be User but rather map[string]interface{} as you experienced).

So yes, you have to pass a pointer to it, but &data will not be what you want. &data will be a pointer to interface, of type *interface{} because makeInstance() returns a value of interface{} type, so data will have that inferred type.

The solution is to change makeInstance() to return a pointer which may be wrapped in an interface{} value, that's ok. And then you may simply pass data to json.Unmarshal(), because the data interface value will hold a value of type *User.

So do:

func makeInstance(name string) interface{} {
    v := reflect.New(typeRegistry[name]) // Don't call Elem() here
    return v.Interface()
}

And in Invoke():

data := makeInstance(signature.Args[0])
json.Unmarshal(body, data)

See related question that was posted just a few hours ago:

Store information/reference about structure

You can see a working demo here: Go Playground.

Upvotes: 3

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